As is known in the art, push-pull amplifier circuits generally include an output stage comprising a pair of output transistors, such as bipolar or Metal Oxide Semiconductor (MOS) transistors, each of which conducts current for a predetermined duration of an excitation cycle. With this arrangement, current is either sourced or sunk by the amplifier output stage depending on the conduction state of the output transistors. The pair of output transistors are complimentary in that one is a p-type transistor (i.e., a pnp or a PMOS device) and the other is an n-type transistor (i.e., an npn or an NMOS device) and the amplifier output signal is provided at the interconnection thereof. For example, when the upper output transistor (i.e., that transistor coupled to a positive supply voltage) is an npn device and the lower output transistor (i.e., that transistor coupled to a negative supply voltage or to ground) is a pnp device, then a push-pull emitter follower amplifier is provided; whereas, when the upper output transistor is a pnp device and the lower output transistor is an npn device, then a common emitter push-pull amplifier is provided.
Amplifier circuits are often categorized by the duration of a sinusoidal excitation, or AC cycle, during which each of the pair of output transistors conducts. For example, in a Class B push-pull amplifier, each of the output transistors conducts for one-half of an AC cycle; whereas, each output transistor of a Class AB push-pull amplifier conducts for more than one-half of a cycle but less than a full cycle.
As is also known, Class B push-pull amplifiers are sometimes susceptible to crossover distortion, a phenomena in which there is a deadband period after one of the pair of output transistors conducts and before the other conducts, during which neither of the output transistors conducts. Stated differently, during the deadband time, the amplifier output stage neither sources nor sinks current. This situation is undesirable since it results in open-loop operation, thereby potentially leading to output oscillations.
One technique for reducing the tendency toward crossover distortion is to provide a quiescent current through each of the pair of output transistors of sufficient magnitude to maintain a forward bias on the gate to source (or base to emitter) diode. Stated differently, this technique requires selecting the quiescent operating point to be slightly above the threshold (or cutoff) point. In this way, the Class B push-pull amplifier essentially operates as a Class AB type amplifier since each output transistor conducts for greater than one-half cycle. With such an arrangement, it is desirable that the quiescent current be relatively accurately controlled since, if the quiescent current is too high, unnecessary power loss may result; whereas, if such current is too low, undesirable crossover distortion, or deadband operation, may result.
One technique for establishing a suitable quiescent current in the case of an emitter-follower push-pull amplifier is shown in FIG. 1. Diodes D1, D2 establish the requisite bias at the base electrodes of transistors Q1, Q2, respectively, in order to forward bias the base to emitter diodes thereof. Preferably, diodes D1, D2 have like characteristics to the base to emitter diodes of the corresponding one of output transistors Q1, Q2, respectively. The current through diodes D1, D2 is controlled by current source I1, to forward bias the diodes D1, D2. In the case where the emitter-follower arrangement of FIG. 1 provides the output stage of a push-pull amplifier, I1 represents a conventional current source, such as is often provided by a current mirror arrangement, and V.sub.i g.sub.m represents a voltage controlled current source where g.sub.m is the transconductance of the preceding amplifier stage (not shown). The current flowing through the series connected complimentary output transistors Q1, Q2 is a "mirrored" version of the controlled, forward-biased current flow through diodes D1, D2. With this arrangement, the base-emitter diodes of the output transistors Q1, Q2 remain forward biased, even under light, or no-load operating conditions.
As is also known, in certain applications, CMOS integrated circuit fabrication is preferable to bipolar technology. For example, mixed signal LSI requires a high degree of functionality and thus benefits from the high integration density of CMOS as opposed to bipolar techniques.